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Abstract:

An oil country tubular good for expansion according to the invention is
expanded in a well. The oil country tubular good for expansion according
to the invention is formed of duplex stainless steel having a composition
containing, in percentage by mass, 0.005% to 0.03% C, 0.1% to 1.0% Si,
0.2% to 2.0% Mn, at most 0.04% P, at most 0.015% S, 18.0% to 27.0% Cr,
4.0% to 9.0% Ni, at most 0.040% Al, and 0.05% to 0.40% N, and the balance
consisting of Fe and impurities, a structure including an austenite ratio
in the range from 40% to 90%. The oil country tubular good for expansion
according to the invention has a yield strength from 256 MPa to 655 MPa,
and a uniform elongation more than 20%. Therefore, the oil country
tubular good for expansion according to the invention has a high pipe
expansion characteristic.

Claims:

1-10. (canceled)

11. An oil country tubular good for expansion in a well formed of duplex
stainless steel having a composition comprising, in percentage by mass,
0.005% to 0.03% C, 0.1% to 1.0% Si, 0.2% to 2.0% Mn, at most 0.04% P, at
most 0.015% S, 18.0% to 27.0% Cr, 4.0% to 9.0% Ni, at most 0.040% Al, and
0.05% to 0.40% N, and the balance consisting of Fe and impurities, and a
structure comprising an austenite ratio in the range from 40% to 90%,said
oil country tubular good having a yield strength from 276 MPa to 655 MPa,
and a uniform elongation more than 20%.

12. The oil country tubular good for expansion according to claim 11,
wherein said duplex stainless steel further contains at most 2.0% Cu.

13. The oil country tubular good for expansion according to claim 11,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 4.0% Mo and at most 5.0% W.

14. The oil country tubular good for expansion according to claim 12,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 4.0% Mo and at most 5.0% W.

15. The oil country tubular good for expansion according to claim 11,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.8% Ti, at most 1.5% V, and at most
1.5% Nb.

16. The oil country tubular good for expansion according to claim 12,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.8% Ti, at most 1.5% V, and at most
1.5% Nb.

17. The oil country tubular good for expansion according to claim 13,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.8% Ti, at most 1.5% V, and at most
1.5% Nb.

18. The oil country tubular good for expansion according to claim 14,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.8% Ti, at most 1.5% V, and at most
1.5% Nb.

19. The oil country tubular good for expansion according to claim 11,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

20. The oil country tubular good for expansion according to claim 12,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

21. The oil country tubular good for expansion according to claim 13,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

22. The oil country tubular good for expansion according to claim 14,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

23. The oil country tubular good for expansion according to claim 15,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

24. The oil country tubular good for expansion according to claim 16,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

25. The oil country tubular good for expansion according to claim 17,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

26. The oil country tubular good for expansion according to claim 18,
wherein said duplex stainless steel further contains one or more selected
from the group consisting of at most 0.02% B, at most 0.02% Ca and at
most 0.02% Mg.

27. Duplex stainless steel used for oil country tubular goods for
expansion, comprising, in percentage by mass, 0.005% to 0.03% C, 0.1% to
1.0% Si, 0.2% to 2.0% Mn, at most 0.04% P, at most 0.015% S, 18.0% to
27.0% Cr, 4.0% to 9.0% Ni, at most 0.040% Al, and 0.05% to 0.40% N, and
the balance consisting of Fe and impurities,said duplex stainless steel
comprising an austenite ratio from 40% to 90%, and having a yield
strength from 276 MPa to 655 MPa and a uniform elongation more than 20%.

28. The duplex stainless steel according to claim 27, further comprising
at most 2.0% Cu.

29. The duplex stainless steel according to claim 27, further comprising
one or more selected from the group consisting of at most 4.0% Mo and at
most 5.0% W.

30. The duplex stainless steel according to claim 27, further comprising
one or more selected from the group consisting of at most 0.8% Ti, at
most 1.5% V, and at most 1.5% Nb.

31. The duplex stainless steel according to claim 27, further comprising
one or more selected from the group consisting of at most 0.02% B, at
most 0.02% Ca, and at most 0.02% Mg.

Description:

TECHNICAL FIELD

[0001]The present invention relates to an oil country tubular good and
duplex stainless steel, and more specifically, to an oil country tubular
good to be expanded in a well and duplex stainless steel to be used for
such an oil country tubular good for expansion.

BACKGROUND ART

[0002]When a well (oil well or gas well) that yields oil or gas is drilled
in general, a plurality of oil country tubular goods called "casings" are
inserted into a well drilled using a drill pipe in order to prevent the
wall of the well from being collapsed. A conventional method of
constructing a well is as follows. To start with, when a well is drilled
for a prescribed distance, a first casing is inserted. Then, when the
well is further drilled for a prescribed distance, a second casing having
an outer diameter smaller than the inner diameter of the first casing is
inserted. In this way, according to the conventional construction method,
the outer diameters of casings to be inserted are sequentially reduced as
the well is drilled deeper. Therefore, as the oil well is deeper, the
inner diameters of casings used in the upper part of the well (near the
surface of the ground) increase. As a result, the drilling area
increases, which pushes up the drilling cost.

[0003]A new technique for reducing the drilling area and thus reducing the
drilling cost is disclosed by JP 7-507610 A and the pamphlet of
International Publication WO 98/00626. The technique disclosed by these
documents is as follows. A casing C3 having a smaller outer diameter than
the inner diameter ID1 of casings C1 and C2 already provided in a well is
inserted into the well. Then, the inserted casing C3 is expanded, so that
its inner diameter is equal to the inner diameter ID1 of the previously
provided casings C1 and C2 as shown in FIG. 1. According to the method,
the casing is expanded inside the well and therefore it is not necessary
to increase the drilling area if the oil well to construct is deep.
Therefore, the drilling area can be reduced. Furthermore, the number of
necessary steel pipes can be reduced because large size casings are not
necessary.

[0004]In this way, the oil country tubular good expanded in a well must
have a uniformly deforming characteristic when expanded (hereinafter
referred to as "pipe expansion characteristic.") In order to obtain a
high pipe expansion characteristic, the deforming characteristic without
local constriction during working is required, in other words, uniform
elongation that can be evaluated by tensile testing must be high.

[0005]As shown in FIG. 1 in particular, in the bell part 10 where casings
vertically placed on each other overlap, the pipe expansion ratio is
maximized. In consideration of the expansion ratio at the bell part, the
uniform elongation of the oil country tubular good for expansion is
preferably more than 20%.

[0006]JP 2005-146414 A discloses a seamless oil country tubular good for
expansion. The structure of the disclosed oil country tubular good
includes a ferrite transformation phase and low temperature
transformation phases (such as bainite, martensite, and bainitic
ferrite), and has a high pipe expansion characteristic. However, the
uniform elongation of each test piece in the disclosed embodiment is not
more than 20% (see JP 2005-146414 A, u-E1 in Tables 2-1 and 2-2).
Therefore, the bell part described above may not deform uniformly.

DISCLOSURE OF THE INVENTION

[0007]It is an object of the invention to provide an oil country tubular
good for expansion having a high pipe expansion characteristic. More
specifically, it is to provide an oil country tubular good having a
uniform elongation more than 20%.

[0008]In order to achieve the above-described object, the inventors
examined the uniform elongation of various types of steel. As a result,
the inventors have found that duplex stainless steel having prescribed
chemical components has a uniform elongation significantly higher than
those of carbon steel and martensitic stainless steel.

[0009]The inventors have further studied and found that in order to
produce an oil country tubular good having a uniform elongation more than
20%, the following requirements must be fulfilled.

[0010](1) The austenite ratio in the duplex stainless steel is in the
range from 40% to 90%. Herein, the austenite ratio is measured by the
following method. A sample is taken from an arbitrary position of an oil
country tubular good for expansion. The sample is mechanically polished
and then subjected to electrolytic etching in a 30 mol % KOH solution.
The etched surface of the sample is observed using a 400× optical
microscope with a 25-grating ocular lens, and the austenite ratio is
measured by a point count method according to ASTM E562.

[0011](2) The yield strength is adjusted in the range from 276 MPa to 655
MPa. The yield strength herein is 0.2% proof stress according to the ASTM
standard. When an oil country tubular good for expansion is kept
as-solution treated, the yield strength is within the above-described
range. Herein, "as-solution treated" means the state in which after the
solution treatment, no other thermal treatment or no other cold working
is carried out except for cold straightening.

[0012]The present invention was made based on the above-described findings
and the invention can be summarized as follows.

[0013]An oil country tubular good for expansion according to the invention
is expanded in a well. The oil country tubular good for expansion
according to the invention is formed of duplex stainless steel having a
composition containing, in percentage by mass, 0.005% to 0.03% C, 0.1% to
1.0% Si, 0.2% to 2.0% Mn, at most 0.04% P, at most 0.015% S, 18.0% to
27.0% Cr, 4.0% to 9.0% Ni, at most 0.040% Al, and 0.05% to 0.40% N, and
the balance consisting of Fe and impurities, and a structure including an
austenite ratio in the range from 40% to 90%. The oil country tubular
good has a yield strength from 256 MPa to 655 MPa, and a uniform
elongation more than 20%.

[0014]Herein, the "uniform elongation" means the distortion (%) at the
maximum load point in a tensile test. The austenite ratio is an austenite
area ratio.

[0015]The duplex stainless steel may further contain at most 2.0% Cu. The
duplex stainless steel may further contain one or more selected from the
group consisting of at most 4.0% Mo and at most 5.0% W. The duplex
stainless steel may further contain one or more selected from the group
consisting of at most 0.8% Ti, at most 1.5% V, and at most 1.5% Nb. The
duplex stainless steel may further contain one or more selected from the
group consisting of at most 0.02% B, at most 0.02% Ca and at most 0.02%
Mg.

[0016]The duplex stainless steel according to the invention is used for
the above-described oil country tubular good for expansion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic view for use in illustrating a new method of
constructing a well that yields oil or gas.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018]Now, embodiments of the invention will be described in detail.

[0019]An oil country tubular good according to an embodiment of the
invention is formed of duplex stainless steel having the following
chemical composition and metal structure. Hereinafter, "%" related to
elements means "% by mass."

[0020]1. Chemical Composition

[0021]C: 0.005% to 0.03%

[0022]Carbon stabilizes the austenite phase. In order to effectively
secure the effect, the C content is not less than 0.005%. Meanwhile, if
the C content exceeds 0.03%, carbide is more easily precipitated, which
lowers the grain boundary corrosion resistance. Therefore, the C content
is from 0.005% to 0.03%.

[0023]Si: 0.1% to 1.0%

[0024]Silicon deoxidizes the steel. In order to secure the effect, the Si
content is not less than 0.1%. Meanwhile, if the Si content exceeds 1.0%,
intermetallic compounds are acceleratingly generated, which lowers the
hot workability. Therefore, the Si content is from 0.1% to 1.0%.

[0025]Mn: 0.2% to 2.0%

[0026]Manganese deoxidizes and desulfurizes the steel and improves the hot
workability as a result. Manganese also increases the solid solubility of
N. In order to effectively secure the effect, the Mn content is not less
than 0.2%. Meanwhile, if the Mn content exceeds 2.0%, the corrosion
resistance is lowered. Therefore, the Mn content is from 0.2% to 2.0%.

[0027]P: 0.04% or less

[0028]Phosphorus is an impurity that promotes central segregation and
degrades the sulfide stress cracking resistance. Therefore, the P content
is preferably as small as possible. Therefore, the P content is not more
than 0.04%.

[0029]S: 0.015% or less

[0030]Sulfur is an impurity and lowers the hot workability. Therefore, the
S content is preferably as small as possible. The S content is therefore
not more than 0.015%.

[0031]Cr: 18.0% to 27.0%

[0032]Chromium improves the carbon dioxide corrosion resistance. In order
to secure sufficient carbon dioxide corrosion resistance for duplex
stainless steel, the Cr content is not less than 18.0%. Meanwhile, if the
Cr content exceeds 27.0%, intermetallic compounds are acceleratingly
generated, which lowers the hot workability. Therefore, the Cr content is
from 18.0% to 27.0%, preferably from 20.0% to 26.0%.

[0033]Ni: 4.0% to 9.0%

[0034]Nickel stabilizes the austenite phase. If the Ni content is too
small, the amount of ferrite in the steel is excessive, and the
characteristic of the duplex stainless steel does not result. The solid
solubility of N in the ferrite phase is small, and the increase in the
ferrite amount causes nitride to be precipitated, which degrades the
corrosion resistance. Meanwhile, an excessive Ni content reduces the
ferrite amount in the steel, and the characteristic of the duplex
stainless steel does not result. In addition, an excessive Ni content
causes a σ phase to be precipitated. Therefore, the Ni content is
from 4.0% to 9.0%, preferably from 5.0% to 8.0%.

[0035]Al: 0.040% or less

[0036]Aluminum is effective as a deoxidizing agent. However, if the Al
content exceeds 0.040%, inclusions in the steel increase, which degrades
the toughness and the corrosion resistance. Therefore, the Al content is
not more than 0.040%.

[0037]N: 0.05% to 0.40%

[0038]Nitrogen stabilizes the austenite phase and also improves the
thermal stability and the corrosion resistance of the duplex stainless
steel. In order to achieve an appropriate ratio between the ferrite phase
and the austenite phase in the steel, the N content is not less than
0.05%. Meanwhile, if the N content exceeds 0.40%, a defect attributable
to a generated blow hole is caused. The toughness and corrosion
resistance of the steel are degraded as well. Therefore, the N content is
from 0.05% to 0.40%, preferably from 0.1% to 0.35%.

[0039]Note that the balance of the duplex stainless steel according to the
invention consists of Fe and impurities.

[0040]The duplex stainless steel for an oil country tubular good for
expansion according to the embodiment further contains Cu in place of
part of Fe if necessary.

[0041]Cu: 2.0% or less

[0042]Copper is an optional element and improves the corrosion resistance
of the steel. However, an excessive Cu content lowers the hot
workability. Therefore, the Cu content is not more than 2.0%. Note that
in order to effectively secure the above-described effect, the Cu content
is preferably not less than 0.2%. However, if the Cu content is less than
0.2%, the above-described effect can be obtained to some extent.

[0043]The duplex stainless steel for an oil country tubular good for
expansion according to the embodiment further contains one or more
selected from the group consisting of Mo and W in place of part of Fe if
necessary.

[0044]Mo: 4.0% or less

[0045]W: 5.0% or less

[0046]Molybdenum and tungsten are optional elements. These elements
improve the pitting corrosion resistance and the deposit corrosion
resistance. However, an excessive Mo content and/or an excessive W
content causes a σ phase to be more easily precipitated, which
embrittles the steel. Therefore, the Mo content is not more than 4.0% and
the W content is not more than 5.0%. In order to effectively secure the
above described effect, the Mo content is preferably not less than 2.0%
and the W content is preferably not less than 0.1%. However, if the Mo
content and the W content are less than the described lower limits, the
above-described effect can be obtained to some extent.

[0047]The duplex stainless steel for an oil country tubular good for
expansion according to the embodiment further contains one or more
selected from the group consisting of Ti, V, and Nb in place of part of
Fe if necessary.

[0048]Ti: 0.8% or less

[0049]V: 1.5% or less

[0050]Nb: 1.5% or less

[0051]Titanium, vanadium, and niobium are optional elements. These
elements improve the strength of the steel. However, if the contents of
these elements are excessive, the hot workability is lowered. Therefore,
the Ti content is 0.8% or less, the V content is 1.5% or less, and the Nb
content is 1.5% or less. In order to more effectively secure the
above-described effect, the Ti content is preferably not less than 0.1%,
and the V content is preferably not less than 0.05%. The Nb content is
preferably not less than 0.05%. However, if the Ti, V, and Nb contents
are less than the above-described lower limits, the above effect can be
obtained to some extent.

[0052]The duplex stainless steel for an oil country tubular good according
to the embodiment further contains one or more selected from the group
consisting of B, Ca, and Mg in place of part of Fe.

[0053]B: 0.02% or less

[0054]Ca: 0.02% or less

[0055]Mg: 0.02% or less

[0056]Boron, calcium, and magnesium are optional elements. These elements
improve the hot workability. However, if the contents of these elements
are excessive, the corrosion resistance of the steel is lowered.
Therefore, the B content, the Ca content, and the Mg content are each not
more than 0.02%. In order to more effectively secure the above-described
effect, the B content, the Ca content, and the Mg content are each
preferably not less than 0.0002%. However, if the B, Ca, and Mg contents
are less than the lower limits, the above-described effect can be
obtained to some extent.

[0057]2. Metal Structure

[0058]The duplex stainless steel that forms an oil country tubular good
for expansion according to the invention has a metal structure including
a ferrite phase and an austenite phase. It is considered that the
austenite phase as a soft phase contributes to improvement of the uniform
elongation.

[0059]The austenite ratio in the steel is from 40% to 90%. Herein, the
austenite ratio is an area ratio measured by the following method. A
sample is taken from an arbitrary position of an oil country tubular good
for expansion and mechanically polished, and then the polished sample is
subjected to electrolytic etching in a 30 mol % KOH solution. The etched
surface of the sample is observed using a 400× optical microscope
with a 25 grating ocular lens, and the austenite ratio is measured by the
point count method according to ASTM E562.

[0060]If the austenite ratio is less than 40%, the uniform elongation is
reduced to 20% or less. Meanwhile, if the austenite ratio exceeds 90%,
the corrosion resistance of the steel is degraded. Therefore, the
austenite ratio is from 40% to 90%. The austenite ratio is preferably
from 40% to 70%, more preferably from 45% to 65%.

[0061]3. Manufacturing Method

[0062]The oil country tubular good for expansion according to the
invention is produced by the following method.

[0063]Molten steel having the above-described composition is cast and then
formed into billets. The produced billet is subjected to hot working and
made into an oil country tubular good for expansion. As the hot working,
for example, the Mannesmann method is carried out. As the hot working,
hot extrusion may be carried out, or hot forging may be carried out. The
produced oil country tubular good for expansion may be a seamless pipe or
a welded pipe.

[0064]The oil country tubular good for expansion after the hot working is
subjected to solution treatment. The solution treatment temperature at
the time is from 1000° C. to 1200° C. If the solution
treatment temperature is less than 1000° C., a σ phase is
precipitated, which embrittles the steel. The yield strength is raised
and exceeds 655 MPa because of the precipitation of the σ phase,
and therefore the uniform elongation is 20% or less. On the other hand,
if the solution treatment temperature exceeds 1200° C., the
austenite ratio is significantly lowered and becomes less than 40%. The
solution treatment temperature is preferably from 1000° C. to
1175° C., more preferably from 1000° C. to 1150° C.

[0065]The oil country tubular good for expansion according to the
invention is in an as-solution-treated state (so-called
as-solution-treated material). More specifically, the tubular good is
used as a product right after the solution treatment without being
subjected to other heat treatment and cold working (such as cold
reduction or pilger rolling) except for cold straightening. In this way,
since the oil country tubular good for expansion according to the
invention is in an as-solution-treated state, and therefore the yield
strength may be in the range from 276 MPa to 655 MPa (40 ksi to 95 ksi).
It is considered that in this way, the uniform elongation exceeds 20% and
a high expansion characteristic is obtained even in a well. Note that if
the yield strength exceeds 655 MPa, the uniform elongation is 20% or
less. The oil country tubular good for expansion needs strength in a
certain level, and the yield strength is 276 MPa or more.

[0066]Note that if cold working is carried out after the solution
treatment, the yield strength exceeds 655 MPa. Therefore, the uniform
elongation is less than 20%.

EXAMPLE

[0067]A plurality of steel products having the chemical compositions in
Table 1 were cast and formed into billets. The produced billets were
subjected to hot forging and hot rolling and a plurality of steel plates
for testing having a thickness of 30 mm, a width of 120 mm, and a length
of 300 mm were formed.

[0069]Steel plates with test Nos. 1 to 23 were subjected to heat treatment
as described in the "heat treatment" column and cold working in Table 1.
More specifically, the steel plates with test Nos. 1 to 11 were subjected
to solution treatment in the temperature range from 1050° C. to
1150° C. ("ST" in the "heat treatment" column in Table 1). The
solution treatment temperature for each of the steel plates is shown in
the "ST temperature" in Table 1. The steel plates with test Nos. 1 to 11
were each a so-called as-solution-treated material without being
subjected to other heat treatment or cold working such as cold reduction
after the solution treatment.

[0070]The steel plates with test Nos. 12 to 20 were quenched at
920° C. and then tempered in the temperature range from
550° C. to 730° C. ("QT" in the "heat treatment" column in
Table 1). The steel plate with test No. 21 was subjected to solution
treatment at a temperature less than 1000° C., and the steel plate
with test No. 22 was subjected to solution treatment at a temperature
higher than 1200° C. The steel plates with test Nos. 21 and 22 are
as-solution-treated materials. The steel plate with test No. 23 was
subjected to solution treatment at 1085° C. followed by cold
drawing.

[0071]Measurement of Austenite Ratio

[0072]For the steel plates of duplex stainless steel with test Nos. 1 to
11 and 21 to 23, the austenite ratio was obtained after the heat
treatment. More specifically, a test piece was taken from each of these
steel plates. The sampled test pieces were mechanically polished and the
polished test pieces were subjected to electrolytic etching in a 30 mol %
KOH solution. The etched surfaces of the samples were observed using a
400× optical microscope with 25 grating ocular lens in 16 fields.
The austenite ratio (%) was obtained for each of the observed fields. The
austenite ratios were obtained by the point count method according to
ASTM E562. The average of the austenite ratios (%) obtained for each of
the fields is given in the "γ" column in Table 1.

[0073]Tensile Testing

[0074]A round bar specimen having an outer diameter of 6.35 mm, and a
parallel part length of 25.4 mm was taken from each of the steel plates 1
to 23 in the lengthwise direction and subjected to a tensile test at room
temperature. The yield strengths (MPa) obtained by the tensile tests are
given in the "YS" column in Table 1, the tensile strengths (MPa) are
given in the "TS" column in Table 1, and the uniform elongations (%) are
given in the "UE" column in Table 1. The 0.2% proof stress according to
the ASTM standard was defined as the yield strength (YS). The distortion
of a specimen at the maximum load point was defined as the uniform
elongation (%).

[0075]Test Result

[0076]With reference to Table 1, the steel plates with test Nos. 1 to 11
each had a chemical composition, a metal structure and a yield strength
within the ranges defined by the invention, and therefore their uniform
elongations all exceeded 20%.

[0077]Meanwhile, the steel plates with test Nos. 12 to 20 were not made of
duplex stainless steel and therefore their uniform elongations were not
more than 20%.

[0078]The steel plate with test No. 21 is made of duplex stainless steel
and has a chemical composition within the range defined by the invention,
but its solution-treatment temperature was less than 1000° C.
Therefore, the yield strength exceeded the upper limit by the invention
and the uniform elongation was not more than 20%. It was probably because
the solution-treatment temperature was low and therefore a σ phase
was precipitated, which raised the yield strength.

[0079]Since the steel plate with test No. 22 exceeded 1200° C., the
austenite ratio was less than 40% and the uniform elongation was not more
than 20%. The steel plate with test No. 23 was not an as-solution-treated
material, but subjected to cold working after the solution-treatment.
Therefore, the yield strength exceeded the upper limit of the range
defined by the invention and the uniform elongation was not more than
20%.

[0080]Although the embodiments of the present invention have been
described and illustrated in detail, it is clearly understood that the
same is by way of illustration and example only of how to carry out the
invention and is not to be taken by way of limitation. The invention may
be embodied in various modified forms without departing from the spirit
and scope of the invention.

INDUSTRIAL APPLICABILITY

[0081]The oil country tubular good for expansion and duplex stainless
steel according to the invention are applicable to an oil country tubular
good and particularly applicable as an oil country tubular good for
expansion in a well.